The present invention relates to non-electrolytic binders and cements having a retardant admixture to prevent hardening of the binder and more particularly to a method for on-demand control setting of such binders and cements using electrical current to defeat the retardant.
Concrete is prepared by mixing cement, water, aggregate, and selected admixtures together to make a workable paste. It is molded or placed as desired, consolidated, and then left to harden. Cement needs moisture to hydrate and cure. The reaction of water with the cement in concrete may continue for many years.
Portland cement consists of five major compounds and a few minor compounds. The composition of a typical Portland cement is:
When water is added to cement, each of the compounds undergoes hydration and contributes to the final concrete product. Only the calcium silicates contribute to strength. Tricalcium silicate is responsible for most of the early strength (first 7 days). Dicalcium silicate, which reacts more slowly, contributes only to the strength at later times.
The equation for the hydration of tricalcium silicate is given by:
Tricalcium silicate+Waterxe2x86x92Calcium silicate hydrate+Calcium hydroxide+heat 
2Ca3SiO5+7H2Oxe2x86x923CaOxc2x72SiO2xc2x74H2O+3Ca(OH)2+173.6 kJ 
Upon the addition of water, tricalcium silicate rapidly reacts to release calcium ions, hydroxide ions, and a large amount of heat. The pH of the pore water quickly rises to over 12 because of the release of alkaline hydroxide (OHxe2x88x92) ions. This initial hydrolysis slows down quickly after it starts resulting in a decrease in heat evolved.
The reaction slowly continues producing calcium and hydroxide ions until the system becomes saturated. Once this occurs, the calcium hydroxide starts to crystallize. Simultaneously, calcium silicate hydrate begins to form. Ions precipitate out of solution accelerating the reaction of tricalcium silicate to calcium and hydroxide ions (Le Chatelier""s principle). The evolution of heat is then dramatically increased.
Dicalcium silicate also affects the strength of concrete through its hydration. Dicalcium silicate reacts with water in a similar manner compared to tricalcium silicate, but much more slowly. The heat released is less than that by the hydration of tricalcium silicate because the dicalcium silicate is much less reactive. The products from the hydration of dicalcium silicate are the same as those for tricalcium silicate:
Dicalcium silicate+Waterxe2x86x92Calcium silicate hydrate+Calcium hydroxide+heat 
2Ca2SiO4+5H2Oxe2x86x923CaOxc2x72SiO2xc2x74H2O+Ca(OH)2+58.6 kJ 
The other major components of Portland cement, tricalcium aluminate and tetracalcium aluminoferrite also react with water. Their hydration chemistry is more complicated as they involve reactions with the gypsum as well. Gypsum is added to slow or retard tricalcium aluminate hydration. Oil field cements contain a small amount of gypsum.
Heat is evolved with cement hydration. This is due to the breaking and making of chemical bonds during hydration. The heat generated is shown in FIG. 1 as a function of time. Stage I hydrolysis of the cement compounds occurs rapidly with a temperature increase of several degrees. Stage II is known as the dormancy period. The evolution of heat slows dramatically in this stage. The dormancy period can last from one to three hours. During this period, the concrete is in a plastic state which allows the concrete to be transported and placed without any major difficulty. This is particularly important for the construction trade who must transport concrete to the job site. It is at the end of this stage that initial setting begins. In stages III and IV, the concrete starts to harden and the heat evolution increases due primarily to the hydration of tricalcium silicate. Stage V is reached after approximately 36 hours. The slow formation of hydrate products occurs and continues as long as water and unhydrated silicates are present.
Water-reducing and set-controlling admixtures are classified by ASTM C 494 into seven types:
The materials that generally are available for use as water-reducing and set-controlling admixtures fall into one of eight general classes:
1. Lignosulfonic acids and their salts
2. Modifications and derivatives of lignosulfonic acids and their salts
3. Hydroxylated carboxylic acids and their salts
4. Modifications and derivatives of hydroxylated carboxylic acids and their salts
5. Salts of the sulfonated melamine polycondensation products
6. Salts of the high molecular weight condensation product of naphthalene sulfonic acid
7. Blends of naphthalene or melamine condensates with other water-reducing or set-controlling materials, or both
8. Other materials, which include: (a) inorganic materials, such as zinc salts, borates, phosphates, chlorides; (b) amines and their derivatives; (c) carbohydrates, polysaccharides, and sugar acids; and (d) certain polymeric compounds, such as cellulose-ethers, melamine derivatives, naphthalene derivatives, silicones, and sulfonated hydrocarbons.
These materials may be used singly or in combination with other organic or inorganic, active, or essentially inert substances to control cement set.
Many of the admixtures used for specific purposes in concrete are used as grouting admixtures to impart special properties to the grout. Oil-well cementing grouts encounter high temperatures and pressures with considerable pumping distances involved. Grout for preplaced aggregate concrete requires extreme fluidity and nonsettling of the heavier particles. Nonshrink grout requires a material that will not exhibit a reduction from its volume at placement. A wide variety of special purpose admixtures are used to obtain the special properties required.
For oil-well cementing grouts, retarders are useful in delaying setting time. Bentonite clays may be used to reduce slurry density, and materials such as barite and iron filings may be used to increase the density. Tile grouts and certain other grouts use materials such as gels, clays, pregelatinized starch, and methyl cellulose to prevent the rapid loss of water. It is standard procedure to add more retardant than is actually needed to allow enough time for placing the grout, sometimes up to 20,000 feet deep in a well to avoid set problems. This procedure sometimes results in a loss over the control of the set, either the set is too rapid due to complex chemistry down hole, or set does not occur as planned requiring waiting days at sometimes millions of dollars of cost per day.
Binders or cements which require electrolytic reactions and acidic conditions to harden are well known in the prior art. For example, a zinc phosphate hydrate cement is produced by combining a metal oxide zinc powder with an acid such as phosphoric acid. When these two components are intermixed, rapid setting occurs and a high quality zinc hydrate cement will be formed.
U.S. Pat. No. 5,252,266 to Brabston et al. teaches electric current induced hardening of binders and cements that rely on electrolysis to effect a pH change around either the cathode or anode thereby changing the surrounding salts to a pH bindable with metal oxides or other additives.
U.S. Pat. No. 5,268,032 to Malone et al. also relies on electrolysis to form an acid or base that reacts with metal oxide to initiate formation of a metal oxide phosphate. Fibers are used to add flexural and tensile strength.
Although these prior art cements have found wide acceptance, they possess certain drawbacks in practical application. Conventional methods require mixing of the cement components prior to placement within a suitable mold. As a result, the intermixed components tend to xe2x80x9csetxe2x80x9d prior to completion of the molding process. This premature hardening of the cement often results in a flawed product or clogged and damaged equipment. In addition, the rapid setting characteristic of the cement requires that all the mixing equipment be fully and completely cleaned immediately after transfer of the cement to the mold.
A disadvantage of electrolytic acid-setting binders and cements is the need to prepare in advance the strong acid for addition to the metal oxide powder. Handling and preparation of such acids can be hazardous and requires great care on the part of the user. Further, articles produced with conventional acid-setting binders or cements are restricted by the type and number of filler materials which can be employed. Time constraints in terms of mixing and the need to avoid premature setting often make the inclusion of fillers within a finished product impractical or will yield a product of inferior quality.
Even though electrolytic acid-setting cements and binders have found wide acceptance within the construction and molding industries, the uncontrolled speed and time at which they set has restricted their expansion into other technologies. A need has therefore existed within the art for non-electrolytic cements or binders which can be controllably hardened on-demand and in so doing avoid the aforementioned problems. These new methods do not rely on electrolysis to initiate chemical set.
A simple means for promoting cement or concrete set on demand that has been chemically retarded with sodium tartrate, a retarder often used in the drilling industry and other construction. One embodiment of the invention adds 3% carbon fiber to the concrete, which enables it to become electrically conductive, 3% sodium tartrate retardant, and copper sulfate which forms a soluble copper tartrate complex in alkaline concrete mixes. Using electricity, the concrete mix anodically converts the retarding tartrate to an insoluble polyester polymer inside the cement pores. The carbon fibers act as a continuous anode surface with a counter electrode wire embedded in the mix. Upon energizing for 2 to 3 hours, the retarding effect of tartrate is defeated by formation of the polyester polymer through condensation esterification thereby allowing the normal set to proceed unimpeded.
Accordingly, it is an advantage of the present invention to provide a method for the controlled hardening of an alkaline-setting concrete thereby preventing premature setting, flaws within the item being cast and clogged or damaged mixing equipment.
Another advantage of the present invention is to provide a method for setting a concrete whereby an alkaline concrete with an electrically conductive admixture is produced in situ thereby eliminating the need for the preparation of a separate acid solution for mixing with the metal oxide cement.
Still another advantage of the present invention is to provide a method for the controlled hardening of an alkaline-setting concrete whereby the mold itself is an operating electrochemical cell.
A still further advantage of the present invention is to provide a method for hardening alkaline-setting concrete and binders whereby the molded end product contains various admixtures.
A still further advantage of the present invention is to provide a method for the controlled hardening of alkaline-setting concretes or grouts whereby the components for producing the alkaline-setting cement are uniformly mixed prior to hardening so as to provide a high quality end product.
Another advantage of the present invention is to provide a method for the controlled hardening of alkaline-setting concrete regardless of ambient temperature.
Further and other advantages of the present invention will become apparent from the description contained herein, read together with the attached claims and figures.